You might give Lord Jeff's a shot and could try calling first. I know that the Beacon Street Star Market in Somerville also used to carry it, but I am more used to seeing the gallon containers. If you want to call around, I might try Johnie's (ask for a manager and they also allow you to email them -- Arlington is fairly helpful), McKinnon's, and that Beacon Shaw's. I think you know this, but some places which do their own roast beef locally use "peeled knuckle" for it.